(1) Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device formed on an SOI substrate and a semiconductor device formed by the same method, and particularly to a method of manufacturing semiconductor device including formation of high concentration impurity diffused region having a function which is the same as that of high concentration impurity buried region and a semiconductor device having the impurity diffused region.
As a semiconductor substrate on which an integrated circuit (IC) is formed, a substrate which is formed as a whole by a single crystal material is usually employed and moreover a substrate formed by providing a thin layer of a semiconductor single crystal (element forming layer) on an insulating layer is also used. In the case of latter substrate, a Si single crystal wafer is often used as a supporting substrate but such substrate is generally called an SOI (Silicon on Insulator) substrate irrespective of the substance of supporting substrate.
In case an IC is formed on an ordinary Si single crystal wafer, since an element and a substrate are separated by pn junction, this junction capacitance often impedes improvement of element characteristics. However, use of SOI substrate provides an advantage that since an element and a substrate are separated by an insulating layer, less influence of parasitic capacitance is generated and moreover adjacent elements are also separated and insulated easily.
But, formation of a high performance bipolar transistor on the SOI substrate results in the following problems. Namely, it is required for obtaining a high performance bipolar transistor to maintain a collector resistance within the required limit and therefore a buried layer having a low resistance is provided usually in the element forming layer in order to satisfy such requirement. However, in manufacturing of an SOI type IC, as will be explained later, since a substrate already forming a buried layer is used, arrangement of such buried layer cannot be set freely and thereby arrangement of elements is extensively restricted.
This problem becomes a large barrier particularly for manufacture of a bipolar type IC. Since, when free layout of transistor is attempted, a general purpose SOI substrate cannot be used, the manufacture must be started from fabrication of SOI substrate which requires extra time and cost. Therefore, it has strongly been requested to develop a method to intentionally form the buried layer in the SOI substrate having no buried layer.
(2) Description of the Related Art
As the most popular method to form a buried layer in a Si single crystal substrate, a high concentration region is formed to the predetermined position on the substrate surface by diffusing an impurity and an element forming layer is then grown thereon by the epitaxial growth method. This method has a merit that a buried layer can be formed in the desired position but allows increase of a couple of processes, namely the selective diffusion and epitaxial growth. Especially, in the SOI substrate, it is not an easy process to control the thickness of an epitaxial layer grown after the selective diffusion process to the required thickness, because the required element forming layer is frequently set to 1 .mu.m or less.
In the conventional method of manufacturing the SOI substrate, two sheets of Si single crystal wafers (hereinafter, called only Si wafer) are bonded with each other through an oxide film. The processes to form an SOI substrate having buried layers by this method are schematically shown in FIGS. 1(a) to 1(d). As shown in FIG. 1(a), the buried layers 16 are formed at the surface of the one Si wafer 10 and moreover the SiO.sub.2 film 12 is formed on the entire part of the surface thereof. (In this process, the buried layer 16 is yet impurity diffused region, but is so called for the convenience of explanation). Formation of such buried layer 16 is usually realized by the ion implantation method but moreover it can also be realized by gas diffusion or solid diffusion method. Thereafter, the SiO.sub.2 film 12 is formed in the thickness of about 1 .mu.m to the entire surface by the thermal oxidation process. The other Si single crystal wafer 11 to be bonded is also shown in FIG. 1(b).
As shown in FIG. 1(c), two sheets of Si wafers 10, 11 are stacked with the surface to which the buried layers 16 are formed provided inside and these wafers are bonded by the thermal process through the SiO.sub.2 film 12. Next, as shown in FIG. 1(d), an SOI substrate having the buried layers 16 at the bottom part of the element forming layer 14 can be obtained by polishing the Si wafer 10 in the side of forming the buried layer to the predetermined thickness.
As explained above, the conventional method to form a SOI substrate having the buried layers includes the Si wafer polishing process. This process is generally different from a processing step called a wafer process and shows bad matching property with the other process. Therefore, it has been considered inadequate that such process is comprised into the IC manufacturing process. Namely, in manufacture of an SOI type IC, it is desirable that manufacture of SOI type substrate including the wafer polishing process is isolated from formation of buried layers.